Speedo inaccuracy

[John1964]

Hi would fitting slightly bigger tyres 

resolve this problem currently running 

225/45/17 would it also increase the ride quality car always feels a little fidgety 

[FROSTYBALLS]

Legally in the EU the indicated speed shown on a speedometer must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading. The indicated speed must not be more than 110 percent of the true speed plus 4 km/h (2.5 mph) at specified test speeds. 

Additionally in the UK, the indicated speed must not be more than 110 percent of the true speed plus at all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually traveling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

So what are you judging the speedometer against?  The speed shown by GPS is an average speed over a given number of points - so although it may be more accurate than a speedometer, it isn't the true speed.

If considering a change of tyres, you need to check what effect the change will have on the speedometer, and using a tyre size calculator should help you avoid making your speedometer under-read (illegal) and possibly reduce the over read.

Eg: https://www.oponeo.co.uk/tools/tyre-size-calculator